1. Field of the Invention
The present invention relates to an image pickup apparatus, such as a camera, having an image stabilization function, and a control method thereof.
2. Description of the Related Art
Image pickup apparatuses have been automated and made multifunctional in every aspect, typically represented by auto exposure (AE) and auto focus (AF), thus enabling anybody to easily accomplish good shooting. In recent years, with the increasing trend toward a reduced size of an image pickup apparatus and a higher magnification optical system, a shake of the image pickup apparatus has been a major cause of deteriorated definition of a shot image. This problem has received attention, and there have been various types of image pickup apparatuses provided with image stabilization functions for compensating for shakes of the image pickup apparatuses.
There are two types of methods for detecting shakes. One type uses an angular velocity sensor or an acceleration sensor to detect a shake of an image pickup apparatus itself. In the other type, a movement of an object is subtracted from an image movement detected from a shot image, and a signal based on the result of the subtraction is detected as the shake of the image pickup apparatus.
Japanese Patent Application Laid-Open No. H10-322584 has proposed a technique whereby the blur signals detected according to the aforesaid two types of methods are combined to implement image stabilization. Japanese Patent Application Laid-Open No. 2007-19571 has proposed to change the weighting in synthesizing the blur information from a sensor and the blur information from a movement vector according to a shooting mode, such as a moving image shooting mode and a still image shooting mode, when implementing the blur stabilization based on the aforesaid two types of methods.
An angular velocity sensor, which is typically used as a shake detection sensor, is capable of detecting a rotational shake of an apparatus itself. However, if the position where the angular velocity sensor is installed is dislocated from the center of rotation, then the shake of the apparatus cannot be accurately detected. In addition, although the apparatus is subjected to vertical and lateral shift shakes, the angular velocity sensor is intrinsically incapable of detecting such shake components. When used for detecting shakes, an acceleration sensor is capable of detecting shift shakes, whereas a plurality of sensors is required to be installed to detect rotational shakes. Furthermore, calculating the amount of a shake from acceleration requires two stages of integration processing. This tends to cause an offset due to a computation error, making it difficult to accurately detect a shake.
Meanwhile, high-speed readout from an image pickup element and higher-speed image processing has permitted shooting at a high-speed frame rate. This has allowed a frequency band, in which a shake can be detected by a movement vector, to be sufficiently wide for detecting a hand shake or a vehicle shake. In detecting a movement vector, an angular velocity sensor is capable of detecting shake components, which are difficult to detect, thus permitting further accurate blur correction. However, in the case of shooting at a low frame rate, the frequency band in which a shake can be detected by a movement vector inconveniently narrows in the shooting at a low frame rate, so that a high-frequency shake, such as a vehicle shake, cannot be corrected. Furthermore, the movement vector requires the processing for determining whether a movement is the movement of an object. Erroneous distinction between an image blur and a movement of the object undesirably leads to unwanted image stabilization.
Accordingly, the combination of the shake information received from the angular velocity sensor and the shake information based on the movement vector must be optimized according to the frequency band, in which the movement vector can be detected, as the frame rate changes.